1. Field of the Invention
The present invention relates to an f-.theta. lens system and a laser scanner using the same, and more particularly to an f-.theta. lens system having a plane skew correction function for a light deflection plane in deflecting a laser beam, which is useful in forming an image such as a laser beam printer (LBP) which effectively focuses a laser spot on a surface to be scanned.
2. Related Background Art
In a prior art laser scanner used for forming an image such as a digital copier or an LBP, a laser beam emitted from a laser light source such as a semiconductor laser is collimated by a collimator lens. The laser beam is then passed through a beam reshaping optical system or a cylindrical lens as required, and it is then directed to a light deflector such as a polygon mirror which rotates at a constant speed. The laser beam reflected and deflected by the deflection plane of the light deflector is focused onto a surface to be scanned such as a photoconductor drum by the f-.theta. lens system to form a laser spot of a predetermined shape, by which the scan plane is scanned.
The f-.theta. lens system used in such a laser scanner has an optical function of focusing the laser beam onto the scan surface and maintaining a constant scanning speed of the laser spot along a main scan direction. In order to correct the ununiformity of scan lines due to a plane skew of the deflection plane of the light deflector, an anamorphic optical system is used in many cases to make the deflection plane and the scan plane conjugate.
In forming an image in the prior art LBP, a diameter of the laser spot on the scan surface may be in the order of 100 .mu.m, and an effective F member F.sub.NO of the f-.theta. lens system which uses the semiconductor laser is approximately 60.about.100. As a result, a depth of focal point on the scan surface is deep and the laser spot diameter is relatively well maintained in the range from a center to a periphery of the scan plane.
In a recent LBP, in order to form a higher resolution image, a laser scanner having an effective F number F.sub.NO of the f-.theta. lens system of less than 40 and the laser spot diameter on the scan surface of less than 50 .mu.m has been proposed.
In such an f-.theta. lens system having the small effective F number F.sub.NO, if an anamorphic optical system is used as one of lenses to provide the plane skew correction function of the deflection plane of the light deflector, the shape of the laser beam is not circular or oval at the periphery of the scan surface but is triangular thereat. This degrades the image quality at the periphery.
U.S. Pat. No. 4,674,825 assigned to the assignee of the present invention discloses a laser scanner which uses an f-.theta. lens system comprising three lenses in total and which keeps the shape of the laser spot well even at the periphery of the scan surface while providing the plane skew correction function of the light deflection plane.
It is an object of the present invention to provide an f-.theta. lens system which is an improvement over the f-.theta. lens disclosed in the U.S. Pat. No. 4,674,825 and which comprises two lenses in total to simplify the overall lens system, keeps the laser spot diameter well even at the periphery of the scan surface, has the plane skew correction function of the deflection plane of the light deflector, has a small eccentricity tolerance in assembling, and is suitable to an image forming apparatus such as an LBP, as well as a laser scanner using such an f-.theta. lens system.
The f-.theta. lens system of the present invention directs a light beam from a light source onto a scan surface through a light deflector to scan the light beam. The f-.theta. lens system comprises two lenses, a first lens having a positive refractive power in a main scan plane and a second lens having a positive refractive power in the main scan plane, arranged in this order as viewed from the light deflector, and a focal length f.sub.M of the overall system in the main scan plane and a focal length f.sub.S of the overall system in a sub-scan plane are different from each other, and the following condition is met: EQU D.sub.2 /f.sub.M .gtoreq.0.1
where D.sub.2 is a distance between the lenses.
In the laser scanner of the present invention, the laser beam from the laser light source is linearly focused to the vicinity of the deflection plane of the light deflector by the focusing system, and the laser beam deflected by the light deflector is directed to the scan surface by the f-.theta. lens to scan the laser beam.
The f-.theta. lens system comprises two lenses, the first lens having the positive refractive power in the main scan plane and the second lens having the positive refractive power in the main scan plane, arranged in this order as viewed from the light deflector. The focal length f.sub.M of the overall system in the main scan plane and the focal length f.sub.S of the overall system in the sub-scan plane are different from each other, and the following condition is met: EQU D/hd 2/f.sub.M .gtoreq.0.1
where D.sub.2 is the distance between the lenses.